In a wireless multi-cell communication system, a WTRU is allowed to move from one cell to another. As the WTRU crosses the boundary of a cell, a handoff is initiated. A soft handoff is a procedure for which communication in one cell is transferred to another cell without interruption. During soft handoff, the same data is received and transmitted between at least two cells.
In order to support soft handoff, the WTRU performs physical measurements on the signals received from a current serving cell and neighboring cells, and reports the measurements to a network upon the occurrence of one or more triggering events. More than one threshold may be established for measurement reporting, depending on the requirements of the system.
For example, in a code division multiple access 2000 (CDMA2000) system, a WTRU measures the strength of pilot signals transmitted by a serving cell and neighboring cells. If the pilot signal strength of any of the cells exceeds a predetermined threshold, the WTRU reports the measurement results to the network when the strength of a neighbor pilot exceeds a predetermined pilot detection threshold, T_ADD, or when the strength of a neighbor pilot exceeds the strength of the pilot of the serving cell by 0.5×T_COMP dB, where T_COMP is a comparison threshold. Based on the measurement results reported by the WTRU, the network decides whether or not to assign a channel to the WTRU in a target cell.
The thresholds used for measurement reporting (e.g., T_ADD and/or T_COMP) are usually established under the assumption that the WTRU uses an omni-directional  antenna, instead of a switch antenna. A switch antenna (i.e., a smart antenna), generates a narrow beam which advantageously increases the signal-to-interference ratio (SIR) in a wireless communication system. However, when the WTRU uses a switch antenna to communicate with a serving base station, the antenna steers to a particular direction, and therefore, the physical measurements on the signals from neighboring cells may be affected by the antenna direction.
FIG. 1 shows a conventional system 100 including a WTRU 102 utilizing a switch beam 106 in communication with a serving cell including a base station (BS) 104a. Since the gain of the switch beam 106 is directed to a particular direction, the signals from neighboring cells (BSs 104b) may not be properly detected even if the neighboring cells are closer than the serving cell.
The received signal power from neighboring cells may be too low in amplitude to trigger the transmission of a measurement report to the network. Thus, a soft handoff may not be triggered which could lead to a higher rate of hard handoffs and increase the number of dropped calls.